1. Technical Field
The present invention relates to a rotary electric apparatus such as on-vehicle alternator, and in particular, to a rotary electric apparatus in which one or more power electronic devices are cooled down with efficiency.
2. Related Art
It is usual that vehicles are equipped with a variety of rotary electric apparatuses such as alternator. Such rotary electric apparatuses adopt semiconductor devices that are vulnerable to heat, which can be seen as IC control devices and power electronic devices such as converter-inverters (semiconductor power converters).
Pursuing more-compact and higher-output rotary electric apparatuses will surely lead to a situation where the temperature to be generated due to heating of the semiconductor devices rises higher. To resolve such a difficulty, it is significant to improve cooling performance for the semiconductor devices.
General cooling techniques which can be adopted by the rotary electric apparatuses include a water cooling technique. This water cooling technique is able to provide a relatively high cooling performance, while the size of a cooling structure for realizing the technique can be made relatively smaller. Such an advantage is however balanced out, because the water cooling technique requires that a considerably large-scale additional device including a piping network be installed therein. This will lead to a rise in manufacturing cost, space occupied by a rotary electric apparatus, and weight of the apparatus. In addition, as is obvious, the water should be reserved at any time, whereby a complex-configured system for reserving the water is needed. Such an additional installment lowers reliability of the entire system in various factors including arrangement of pipes, connections thereof, and fitting the pipes against vibration.
There are other drawbacks as to the water cooling technique. When the semiconductor devices are cooled down by water, it is required to additionally adopt a heat-conductive cooling structure electrically isolated from the cooling medium (water). This adoption will increase transitional thermal resistance. Accordingly, larger fluctuations of thermal load cause an overshoot in the temperature rise, giving rise to a problem that the semiconductor devices may cause thermal deviation. It can therefore be concluded that, though requiring a relatively large-scale and high-cost configuration, the water cooling technique is unable to provide higher cooling performance. There is also another drawback that the water cooling type is lower in the transitional cooling characteristic than an air cooling type of mechanism, which will then be described.
As a first type of air cooling mechanism whose transitional cooling characteristic is higher is proposed by Japanese Patent Laid-open publication No. 5-219685, in which air ducts are piped to intake cool air from a cool air apace in which there is relatively lower temperature air, such as fresh air, and an electric fan is placed in the air ducts. The electric fan is placed to hold or amplify an amount of cooling air so that the loss of air resulting from resistance on the ducts to the flowing air is maintained even at lower-speed revolutions of a vehicle.
A second type of air cooling mechanism is also proposed by Japanese Patent Laid-open publication No. 8-84471. In the proposal, it is first noted that the control of the electric fan at lower-speed revolutions involves a temperature sensor and a speed controller, and then a practical resolution to such a situation is provided. The practical resolution is a technique of connecting a motor to a filed winding DC-exited to control power to be generated.
For the foregoing two types of air cooling mechanisms, it can be expected to have an effect as high as the water cooling type in improving the cooling performance. However, there are still some difficulties to be solved concerning the foregoing two types of air cooling systems. The first one requires that an electric motor for larger ducts and a larger-scale fan be installed, while the second one requires to have a temperature sensing circuit, an activation determining circuit operating based on a sensed temperature outputted from the sensing circuit, and a motor drive/control circuit which is usually high-cost and complex in its configuration. In this way, though a cooling performance as high as that provided by the water cooling type may be obtained, there still remain the problems deformed from the problems that the water cooling type has faced.